1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method and, more particularly, a semiconductor device manufacturing method including a step of growing a local oxidation of silicon (LOCOS) layer on a surface of a semiconductor substrate by using a silicon nitride film as a mask.
2. Description of the Prior Art
A LOCOS layer which is formed on a surface of a semiconductor substrate is employed as a field isolation film for isolating respective devices. A thickness of the field oxide layer is formed differently according to respective portions of device isolation.
When the field oxide layers which have different thicknesses in a plurality regions on the surface of the semiconductor substrate are formed, the thickness of the field oxide layers can be controlled according to a method shown in FIGS. 1A to 1D, for example.
First, as shown in FIG. 1A, a silicon oxide film acting as a pad 2 is formed by thermally oxidizing a surface of a silicon substrate 1. In turn, a silicon nitride layer 3 is formed on the pad 2 by CVD. This silicon nitride layer 3 is formed to prevent oxidation of the surface of the silicon substrate 1. Then, a first window 3a is formed over a first region A, in which a first field oxide layer is to be formed, by patterning the silicon nitride layer 3 by means of photolithography.
Then, as shown in FIG. 1B, a first field oxide layer (LOCOS layer) 4 is formed by oxidizing the surface of the silicon substrate 1 below the first window 3a. This oxidation is the first oxidation step of forming a field oxide layer. For example, this oxidation is performed by heating the surface of the silicon substrate 1 up to 1000.degree. C. and supplying oxygen to the surface of the silicon substrate 1 via the first window 3a of the silicon nitride layer 3. In addition, upon supply of the oxygen, chlorine is added at about 0.25 flow rate percents in the course of oxidation. Then, supply of oxygen and chlorine is suspended, then the temperature of the silicon substrate 1 is increased up to 1125.degree. C. Then, after the temperature of the silicon substrate 1 has been stable, the silicon substrate 1 is oxidized in an oxygen atmosphere to which chlorine is added at 0.05 flow rate percents, whereby a thickness of the first field oxide layer 4 is increased.
During such oxidation, a surface of the silicon oxynitride layer 3 is oxidized slightly to thus form a silicon oxynitride film 5.
Then, as shown in FIG. 1C, second windows 3b are formed over second regions B, in which second field oxide layers are to be formed respectively, by patterning the silicon nitride layer 3 by means of second photolithography.
Then, as shown in FIG. 1D, second field oxide layers (LOCOS layer) 6 are grown by oxidizing the surface of the silicon substrate 1 below the second windows 3b and simultaneously the thickness of the first field oxide layer 4 below the first window 3a is increased. This oxidation is a second oxidation step which is performed under the same conditions as the first oxidation step.
As described above, according to patternings of the silicon nitride layer 3 and oxidations of the silicon substrate 1 both carried but twice, two types of field oxide layers 4, 6 having different thickness can be formed in different regions A, B on the surface of the silicon substrate 1.
However, according to above mentioned steps, as shown in FIG. 2, sometimes a projection 6a is generated on a part of top ends of bird's beak of the second field oxide layer 6.
For this reason, when a thin silicon oxide film 7 is formed by thermally oxidizing the surface of the silicon substrate 1 again after the silicon nitride layer 3 and the pad 2 have been removed on the substrate 1, a thickness of such thin silicon oxide film 7 is made locally small on the projection 6a formed on the surface of the bird's beak.
Therefore, if the silicon oxide film 7 is employed as a tunnel insulating film, carriers are apt to leak from such locally thinned area of the silicon oxide film 7 on the projection 6a.
Such generation of the projection on the bird's beak of the LOCOS, as mentioned above, has been found by the inventors of the present invention. It may be supposed that such generation is due to stress in the silicon oxynitride layer 5 which is formed on the surface of the silicon nitride layer 3. More particularly, when the window 3b is formed in the silicon nitride layer 3 on which the silicon oxynitride layer 5 has been formed, stress is applied to the silicon nitride layer 3 because of stress of the silicon oxynitride layer 5, and then a crack 3c shown in FIG. 3 is easy to be generated on a side surface of the silicon nitride layer 3 exposed from the window 3b. Therefore, if the surface of the silicon substrate is oxidized via the window 3b later, silicon oxynitride is also generated in the silicon nitride layer 3 along the crack 3c, so that such silicon oxynitride generated in the silicon nitride layer 3 is combined integrally with the bird's beak of the LOCOS layer 6. As a result, after the silicon nitride layer 3 has been removed, such silicon oxynitride still remains as the projection 6a in vicinity of the top ends of the bird's beak.